1. Field of the Invention
This invention relates to musical tone synthesis and in particular is concerned with an improvement for systems producing an extended range of harmonics.
2. Description of the Prior Art
Digital musical tone generators are being designed with an ever increasing extension in the maximum number of harmonics. Large numbers of harmonics can present some system logic speed problems. It is apparent that it is not economical to generate tones with large number of harmonics for the entire range of keyboard frequencies. If the number of harmonics is kept at a constant number then the overtone frequencies for the higher octaves fall far above the upper frequency threshold of the listeners.
In U.S. Pat. No. 4,085,644 there is described a polyphonic tone synthesizer in which a master data set of numerical values is computed and stored in a main register from which it is transferred to note registers of a plurality of tone generators. The master data set defines the amplitudes of equally spaced points for a period of the audio waveform of the musical tones being generated. Each tone generator receives the data words in the master data set and transfers them to a digital-to-analog converter at a rate corresponding to an actuated keyboard switch which is assigned to a tone generator.
One of the features of the polyphonic tone synthesizer, as described in the above-identified patent, is that the transfer of successive words from the master data set in the main register to an individual note register in the respective tone generators is synchronized with the transfer of data words from the note register to the digital-to-analog converter. This feature permits the master data set defining the waveform to be recomputed and loaded in the respective tone generators without interrupting the generation of musical tones. The rate at which the waveform can be varied as a function of time is limited by the length of time required for a computation cycle during which the master data set is generated and by the length of time required to transfer the data from the main register to the note registers in each of the tone generators which have been assigned to actuated keyswitches.
In U.S. Pat. No. 4,231,278 a master data set computing subsystem for the tone generator described in U.S. Pat. No. 4,085,644 is described which adaptively computes the set of master data set points in response to values of preselected harmonic coefficients. The computation subsystem advances past all harmonic coefficients of smaller value than a selected threshold value thereby reducing the required computation time and making the musical instrument improve its capability of generating time variant tonal changes.